1. Field of the Invention
This invention pertains to gas turbine engines and pertains more particularly to an improved accessory mechanical drive for transmitting mechanical power radially outwardly from the gas turbine drive shaft to an accessory gear box.
2. Description of the Prior Art
Gas turbine engines typically include an accessory gear box normally mounted in its own casing at the exterior of the gas turbine engine casing. The gear box is mechanically interconnected for transmittal of motive power to a primary central drive shaft of the engine. Normally, starting power for the gas turbine engine is delivered from the accessory gear box through an accessory mechanical drive to rotate the central drive shaft. Then during normal engine operation, power is transmitted from the central drive shaft to the accessory gear box for driving hydraulic, electrical and pneumatic components for generating accessory power. It is necessary that such an accessory mechanical drive operate at extremely high speeds, e.g. 20,000 to 40,000 rpm. Accordingly, it is important that the shafting of the accessory mechanical drive, often referred to as a tower shaft, be of minimum weight and maximum stiffness to avoid critical speed problems.
Economies of operation of a gas turbine engine are also reflected in the simplicity and ease of assembly/disassembly for maintenance purposes. However, in many gas turbines the removal of the accessory mechanical drive requires full disassembly or removal of the accessory gear box.
Various arrangements have been contemplated in the prior art to provide an accessory mechanical drive which may be disassembled without removal of the accessory gear box. These arrangements are replete with operational problems due to the very high speed of operation thereof. For example, arrangements of the prior art include multiple-piece, telescoping tower shafts for the mechanical drive for the purpose of simplifying maintenance. Other arrangements include multiple-piece tower shafting wherein there is adequate space within the gas turbine engine for access to the midspan portion of the tower shaft. Other arrangements contemplated by the prior art have included arrangements of threaded sleeves associated with the accessory mechanical drive tower shaft to allow radial displacement thereof during engine assembly/disassembly. Other methods have contemplated inclusion of bolts with self locking threads which may be accessed to assist in removal of the tower shaft.
A common problem with all known prior art arrangements is that each adds complexity and weight to the tower shaft which is rotating at very high speeds. This impacts the weight and stiffness thereof. Drive joints, introduction of more rotating mass with the tower shaft, bolts, or threaded sleeves--all are arrangements which distract from the basic requirements of the tower shaft of minimum weight and maximum stiffness.
Characteristically the prior art arrangements physically lock the tower shaft of the accessory gear drive against radial movement. In some arrangements the tower shaft is physically secured to the drive train associated with the central drive shaft, or secured with the drive train associated with the accessory gear box. In some arrangements, the tower shaft is secured against radial movement by physically locking it to the engine casing through an appropriate bearing arrangement. Such securement of the tower shaft against radial movement either dramatically complicates engine assembly/disassembly, or adds to the weight of the rotating components associated with the tower shaft, or does both.